1. Field of the Invention
The present invention relates to a higher harmonic control apparatus for a helicopter rotor, for controlling the helicopter rotor so as to suppress the rotor vibration by changing a pitch angle of the helicopter rotor at a frequency which is an integer multiple of a rotor rotating frequency, to reduce noises which are generated by the rotation of the helicopter rotor.
2. Description of the Related Art
FIG. 7 shows a part of a prior art control system for a helicopter. FIG. 7A is a schematic view showing a control system 1 which is not provided with a higher harmonic control apparatus, FIG. 7B is a schematic view showing a control system 3 which is provided with an HHC actuator 2 as a higher harmonic control apparatus, and FIG. 7C is a schematic view showing a control system 5 which is provided with an IBC actuator 4 as a higher harmonic control apparatus. The HHC actuator 2 is an actuator having a higher harmonic control system. Hereinafter, such an actuator is abbreviated as an HHC actuator. The IBC actuator 4 is an actuator having an individual blade control (IBC) system. Hereinafter, such an actuator is abbreviated as an IBC actuator.
Referring to FIG. 7A, a helicopter rotor (hereinafter simply referred to as a rotor) 6 is caused to rotate by the torque transmitted through a rotor shaft 8 from a motor (not shown). The rotor 6 comprises a plurality of blades 7 (for simple illustration, only one blade is shown in FIG. 7). The pitch angle .theta. of the blade 7 is controlled by a control actuator 10 via swash plates 9. As a result, the lift caused at the time when the rotor 6 is rotated is controlled, and the helicopter is maneuvered.
When the rotor 6 is rotated, an external force due a air enveloping the blade 7 acts on the blade 7. The external force is periodically varied in one revolution of the rotor 6, so that the external force serves as an excitation force which vibrates the fuselage of the helicopter (not shown). The generated vibration may adversely affect gauges, cause difficulty in reading gauges, make passengers feel uneasy, and cause fatigue of the fuselage architecture. These adverse effects may become sources of various problems.
In order to prevent vibration of the fuselage which may cause such problems, a tendency to mount a higher harmonic control apparatus in a helicopter rotor is recently seen. Among higher harmonic control apparatuses, apparatuses which actively perform the vibration control include the HHC actuator 2 shown in FIG. 7B, and the IBC actuator 4 shown in FIG. 7C. Each of the actuators is a higher harmonic control actuator for changing the pitch angle .theta. of the blade 7 at a frequency which is an integer multiple of a rotation frequency of the rotor 6, i.e., at a higher harmonic, and for controlling the external force acting on the blade 7, thereby reducing the vibration of the fuselage.
The HHC actuator 2 is disposed below the swash plates 9, and collectively controls the pitch angles .theta. of the blades 7 via the swash plates 9. The IBC actuator 4 is disposed above the swash plates 9, and individually controls the pitch angles .theta. of the blades 7.
Other examples of prior art are described in, for example, Japanese Unexamined Patent Publications JPA 63-308204 (1988), 64-83906 (1989), and 2-113101(1990).
In such prior art examples, the higher harmonic control is actively performed by using a high-response HHC actuator 2 or a high-response IBC actuator 4. It is assumed that the actuator 2 or 4 is designed under the same load conditions as those for the control actuator 10. If the pressurizing areas of the actuators are equal to each other, the amounts of oil are proportional to the operating frequency of the actuators. The HHC actuator 2 or the IBC actuator 4 operates at a higher harmonic (a higher frequency). For these reasons, the amount of hydraulic fluid required for the actuator 2 or 4 to operate is several times as large as that required for the control actuator 10 to operate. As a result, the hydraulic supply has an increased size, and occupies a larger area, and moreover the weight of the fuselage is significantly increased.
The stall of the blades 7 of the rotor 6 which occurs in the flight can be recognized by a pilot because of the increase in vibration of the fuselage. In the case where the HHC actuator 2 or the IBC actuator 4 is mounted on the helicopter, however, there is a drawback fear that the actuator 2 or 4 may damp the vibration caused by the stall. In such a case, it may take a long time for the pilot to recognize the stall of the blades 7.